1. Field of Endeavor
The present invention relates to industrial applications of lasers and more particularly to laser peening of metals.
2. State of Technology
Improving the strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his weapons and tools. Since the 1950s shot peening has been used as a means to improve the fatigue properties of metals. Another method of shock processing involves the use of high explosive materials in contact with the metal surface. The use of high intensity laser outputs for the generation of mechanical shock waves to treat the surfaces of metals has been well known since the 1970s. The laser shock process can be used to generate compressive stresses in the metal surfaces adding strength and resistance to corrosive failure. Lasers with pulse outputs of 10 to 100 J and pulse durations of 10 to 100 ns are useful for generating inertially confined plasmas on the surfaces of metals. These plasmas create pressures in the range of 10,000 to 100,000 atmospheres and the resulting shock pressure can exceed the elastic limit of the metal and thus compressively stress a surface layer as deep or deeper than 1 mm in the metals.
The article, xe2x80x9cBlasts of Light to Strengthen Metals,xe2x80x9d Science and Technology Review, October 1998, provides the following description: xe2x80x9cAt first glance, it would seem that bombarding a metal part with an intense stream of tiny metal or ceramic balls might not be the best approach for making that part more resistant to cracking and corrosion. And yet, shot peening is a tried-and-true technique for strengthening metals. Now a team of Lawrence Livermore researchers, in tandem with colleagues at New Jersey-based Metal Improvement Co. Inc., have replaced the tiny balls with short-lived, repetitive blasts of light from a reliable, high-powered laser.xe2x80x9d
U.S. Pat. No. 4,401,477 for laser shock processing, by Clauer et al, patented Aug. 30, 1983, provides the following description xe2x80x9c. . . constitutes an improvement of the U.S. Pat. No. 3,850,698 which is the basic method of laser shock processing a solid target with a radiation pulse of high power generation, the disclosure of which is herein incorporated by reference. The patented invention is particularly useful when the target material is sufficiently thick so that the stress wave does not penetrate to the back surface of the target material. Their more recent invention involved xe2x80x9cthe mounting of a trapping material (hereinafter referred to as a momentum trap) that is placed along the back surface of the metal substrate to be laser shock processed. It is important that the trap and the substrate have substantially the same acoustical impedance. Other material properties that are important are a matching of material densities, and sound speed. Preferably, the trap and the substrate are the same material, so that no matching is needed. The trap is placed against the back surface of the substrate, and mineral oil can be used there between to enhance the coupling of the materials. A spring-loaded, disk-shaped trapping material is placed against the substrate. An overlay material that is substantially transparent to laser radiation (e.g.xe2x80x94fused quartz, acrylic, water) is placed in direct contact with the front surface of the specimen. Initially the specimen is coated with a thin layer of paint, preferably black, to enhance absorption of the laser radiation and to protect the surface from melting by the radiation. A high power, pulsed laser emits a high intensity pulse of short duration radiation that passes through the overlay material, is absorbed by the paint, produces a high pressure condition at the front surface of the metal substrate, thereby driving a high amplitude stress wave into and through the metal substrate and into the momentum trap. The surface of the paint is vaporized when it is struck by the laser radiation causing an extremely high pressure near the surface when the vaporized gas is trapped between the specimen surface and the transparent overlay. At the back surface of the target specimen the stress wave passes into the momentum trap and is reflected back from the back surfaces. However, the momentum imparted to the trap by the reflected wave causes the spring loaded trap to break away and disconnect from the specimen after the stress wave is reflected from the back surface of the momentum trap and strike the common surface of the specimen and the trap. The momentum trap thereby carries away the tensile wave which would produce distortion in the specimen. Another means of absorbing the stress wave after it has passed through the metal substrate involves the use of a large mass of material, having a long dimension in the same direction as the stress wave, and having substantially the same acoustical impedance as the metal substrate. Being in direct contact with the metal substrate, and mounted so that the stress wave passes through the long dimension of the material mass, the stress wave is considerably weakened when it is reflected back to the common surface of the mass and the substrate. Copending patent application Ser. No. 334,612, now European Patent No. 085278A1 for split beam method of altering material properties, by Clauer et al, issued Aug. 10, 1983, discloses the use of a split beam to simultaneously laser shock the opposing two sides of the target material. However, the split beam method has several inherent disadvantages. 1. To shock process two sides simultaneously requires that the laser generate twice the power as for only doing one side. High powered lasers are extremely expensive and such cost may be prohibitive. 2. The split beam method requires the use of precisely calibrated and positioned lenses and mirrors. This precision may be difficult to achieve in a high production manufacturing environment. 3. In many applications only one side of the metal substrate is subject to fatigue, so there is no need to use the split beam method. In other applications, it is not possible to have line of sight access to both sides of the fatigue critical region for split beam processing.xe2x80x9d This description of the process for simultaneous dual sided peening thin sections fails to adequately recognize or address the problem of internal spalling.xe2x80x9d
U.S. Pat. No. 5,531,570 for distortion control for laser shock peened gas turbine engine compressor blade edges, by Mannava et al, patented Jul. 2, 1996, provides the following description, xe2x80x9cA method for counteracting distortion of the airfoil caused by laser shock peening a gas turbine engine compressor metallic airfoil along its leading and/or trailing edge to form laser shock peened surfaces extending radially along at least a portion of the edges with a region having deep compressive residual stresses imparted by laser shock peening (LSP) extending into the airfoil from the laser shock peened surfaces.xe2x80x9d Although this patent discusses distortion of the part shape, it also fails to address an effective approach to peening of thin components without internally spalling.
The present invention provides a system for altering the properties of a metal piece having first and second surfaces by a process of laser peening. In an embodiment of the invention, the properties of a metal piece having first side and a second side are altered by laser peening the piece on the first side using an acoustic coupling material operatively connected to the second side. In another embodiment of the invention, laser peening is employed from one side at a time with a material on the back side that couples a portion or all of the shock wave out of the metal. Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description and by practice of the invention.